This proposal focuses on identifying functional deficits in motoneurons as they degenerate in a mouse model of ALS, based on the recently published and preliminary results indicating that motoneuron properties that normally specify their activation patterns may play a key role in their degeneration. We focus especially on motoneuron size. The motoneuron normally functions as the central component of a motor unit, which consists of the motoneuron, its axon and the muscle fibers innervated. Thus size involves not just the cell body but also the dendrites (which reflect number of inputs) and axon terminal branches (which is proportional to number of innervated muscle fibers). Normally, motoneurons are activated from small to large: type S motor units are small in terms of motoneuron anatomy and number of muscle fibers, all of which are slow. Progressive larger and faster motor units follow (type FR and FFs). Yet studies of the denervation of muscle fibers in the periphery in a standard animal model of ALS, the mutant SOD1 mouse, indicate that initial failure to generate force occurs in the opposite sequence: FF > FR >S, i.e. from large to small. This reverse sequence suggests excess size is a deficit that contributes to degeneration and indeed we have recently been surprised to find that mutant SOD1 motoneurons began to grow excessively at a very young age, before 10 days of birth. This is long before the first FF motor units begin to fail in force generation (about 50 days) and even longer before classic symptom onset (90 days). Remarkably, the intrinsic electrical properties of these larger cells are also distorted, potentially leading to a combination of metabolic and excitotoxic stress. In addition, changes in the structure of input could occur. To investigate the relations between size, intrinsic excitability and synaptic input requires intracellular study of mouse motoneurons in the adult state. We have developed 3 new preparations that allow the first intracellular studies of motoneuron in the adult state for sacral lumbar and brainstem motoneurons. Two are in vitro, allowing systematic drug studies while one is in situ, allowing direct comparison of motoneuron electrical properties to its mechanical properties. Thus the in situ prep studies will identify the properties of the motoneuron as it undergoes force failure. Aim 1 uses the in situ preparation to test the hypothesis that excess size predicts the pattern of force failure. Aim 2 uses an in vitro sacral cord preparation to asses whether there is parallel upregulation in intrinsic electrical properties and inputs to match the distortion in size, while Aim 3 uses brainstem slice to see if these smaller motoneurons undergo the same pattern. In Aim 4, chronic drug administration is used to determine if alterations in electrical properties cause changes in size. Overall, this work constitutes a new approach to study of mechanisms of ALS.